glPixelZoom specifies values
for the $x$ and $y$ zoom factors. During the execution of glDrawPixels or
glCopyPixels, if ($xr $, $yr $) is the current raster position, and a given
element is in the $m$th row and $n$th column of the pixel rectangle, then
pixels whose centers are in the rectangle with corners at

($xr ~+~ n cdot
"xfactor"$, $yr ~+~ m cdot "yfactor"$)

($xr ~+~ (n+1) cdot "xfactor"$,
$yr ~+~ (m+1) cdot "yfactor"$)

are candidates for replacement. Any pixel
whose center lies on the bottom or left edge of this rectangular region
is also modified.

Pixel zoom factors are not limited to positive values.
Negative zoom factors reflect the resulting image about the current raster
position.